Each large Class 8 over the highway truck and tractor vehicle typically is equipped with three brake system air tanks. The first is a so called "wet tank" which is provided to condense, trap and collect water from water vapor entrained in compressed air provided for a vehicle's brake system. The second and third tanks are independent tanks to provide primary and backup supplies of air for the vehicle's brakes.
Secure yet accessible mounting of these three tanks has been something of a problem. They are relatively bulky, so that relatively large volumes of limited space between rails of a vehicle frame are taken up by them. In addition, they must be positioned such that there is access for connection and service of air conduits to and from them, as well as valving for the brake system and typically with a highway tractor conduits for connection to brake lines of a coupled trailer.
Space within a vehicle frame is at a premium because not only must it contain and support the three air tanks and various associated service lines, but other components such as fuel tanks, fuel and electrical lines and in some cases hydraulic lines and systems, as well as the drive train and accessories.
Class 8 vehicles also typically carry three or four heavy duty batteries for such purposes as providing adequate cranking power to start a large diesel engine and providing adequate power for lighting and accessories. Accordingly, a typical Class 8 vehicle is also equipped with a sturdy battery box for securely mounting and housing the vehicles batteries. Such a battery box also occupies substantial space further contributing to the problem of locating all necessary and desired vehicle components.
Ideally all components are mounted accessibly and at the same time positioned to leave adequate room for a comfortable operating compartment and an engine compartment of sufficient space to afford access to the engine and other vehicle components such as power steering and air conditioning units which are typically located in engine compartments. Further, all of the vehicle components must be sufficiently nested within or otherwise supported by the frame to allow adequate road clearance and to minimize interference with air currents generated by a moving vehicle. In short, they must be positioned to keep vehicle wind drag to an absolute minimum for economy which is vital to profitable vehicle operation.